Balance between nitrogen use efficiency and cadmium tolerance in Brassica napus and Arabidopsis thaliana

The transmembrane transport of NO3- and Cd-2(+) into plant cell vacuoles relies on the energy from their tonoplast proton pumps, V-ATPase and V-PPase. If the activity of these pumps is reduced, it results in less NO3- and Cd-2(+) being transported into the vacuoles, which contributes to better nitrogen use efficiency (NUE) and lower Cd-2(+) tolerance in plants. The physiological mechanisms that regulate the balance between NUE and Cd-2(+) tolerance remain unknown. In our study, two Brassica napus genotypes with differential NUEs, xiangyou 15 and 814, and Atcica-2 mutant and AtCAX4 over-expression line (AtCAX4-OE) of Arabidopsis thaliana, were used to investigate Cd2+ stress responses. We found that the Brassica napus genotype, with higher NUE, was more sensitive to Cd2+ stress. The AtCAX4-OE mutant, with higher Cd2+ vacuolar sequestration capacity (VSC), limited NO3- sequestration into root vacuoles and promoted NUE. Atcica-2 mutants, with decreased NO3- VSC, enhanced Cd2+ sequestration into root vacuoles and conferred greater Cd2+ tolerance than the WT. This may be due to the competition between Cd2+ and NO3- in the vacuoles for the energy provided by V-ATPase and VP-Pase. Regulating the balance between Cd2+ and NO3- vacuolar accumulation by inhibiting the activity of CLCa transporter and increasing the activity of CAX4 transporter will simultaneously enhance both the NUE and Cd2+ tolerance of Brassica napus, essential for improving its Cd2+ phytoremediation potential.